The present invention relates to acidic, aqueous compositions for forming zinc phosphate coatings on metal surfaces, and more specifically to a composition for forming an improved zinc phosphate coating that provides excellent corrosion resistance and paint adhesion on a metal substrate. The improved zinc phosphate coating is particularly suitable as an undercoat for cationic electrodeposition. Heretofore, zinc phosphate coatings have been applied to metal surfaces as an undercoat in order to maximize the properties of coats formed by electrodeposition, generally anionic electrodeposition. In recent years, the growing demand for higher corrosion resistance of the coats has brought about an innovation in the electrodeposition process. Since about 1977, cationic electrodeposition paints having improved corrosion resistance have been developed and put into practice.
In order to make best use of the high corrosion resistance of cationic electrodeposition paints, it is necessary to form a zinc phosphate coating over the object to be painted. Our research into an undercoat that is suitable for cationic electrodeposition has uncovered the fact that the zinc phosphate coats that have been used as an undercoat for anionic electrodeposition exhibit suprisingly poor adhesion to the cationic electrodeposition paints, frequently resulting in poor corrosion resistance due to the breakdown of adhesion.
The causes of this adhesion breakdown are unknown so far, but we theorize, in view of the reaction mechanism for electrodeposition, that the electrodeposition bath in the vicinity of the object being treated becomes considerably acidic during the flow of electric current in anionic electrodeposition, while it becomes alkaline to a considerable degree in cationic electrodeposition so that the undercoats for cationic electrodeposition must exhibit good alkali resistance. The conventional zinc phosphate coatings, however, do not exhibit such alkali resistance. Another cause of the adhesion breakdown may be that, when corrosion takes place under a coating, oxygen is reduced to form an alkali. Thus, when the coating has a low alkali resistance, the coating surface is dissolved, thereby lowering the adhesion to the cationic electrodeposition paint (which has a large internal cohesive power). This may cause an adhesion breakdown, resulting in poor corrosion resistance.
Our study of undercoats capable of imparting alkali resistance to zinc phosphate coatings has led to the present invention, a composition containing a complex fluoride ion.
Until now, many kinds of coating solutions containing a complex fluoride ion have been disclosed, such as, for example, those disclosed in Japanese Patent Publication Nos. 4324/1965; 7129/1967; 12130/1967; and 14223/1975, and Japanese Patent Disclosure Nos. 28337/1973 and 140237/1978.
In Japanese Patent Publication Nos. 4324/1965 and 12130/1967 the object to be coated is a plate of zinc or zinc plated steel, and applied paints are thermosetting solvent type, not cationic electrodeposition. Phosphate coats formed over iron or steel surfaces according to such disclosures, when used as an undercoat for cationic electrodeposition, have poor adhesion to paint films and poor corrosion resistance.
In Japanese Patent Publication Nos. 7129/1967 and 14223/1975, the same coating solution can be used to process both iron and zinc materials, including iron and steel. Applied paints, however, are thermosetting solvent type, not cationic electrodeposition. Phosphate coats formed according to such disclosures, when used as an undercoat for cationic electrodeposition, provide poor paint film adhesion and poor corrosion resistance, and thus are commercially unacceptable.
In Japanese Patent Disclosure Nos. 28337/1973 and 140237/1978, the same treating liquid can be used to process iron, zinc and aluminum materials, including iron and steel. However, the applied paints of Japanese Patent Disclosure No. 28337/1973 are thermosetting solvent type, not cationic electrodeposition. Phosphate coats formed according to that disclosure, when used as an undercoat for cationic electrodeposition, provide poor paint film adhesion and poor corrosion resistance, and thus are commercially unacceptable.
Japanese Patent Disclosure No. 140237/1978 is concerned with undercoats for anionic electrodeposition. Phosphate coats formed according to that disclosure have poor alkali resistance, and, when used as an undercoat for cationic electrodeposition, the coatings of the disclosure provide poor paint film adhesion and poor corrosion resistance.
Furthermore, Japanese Patent Publication Nos. 12130/1967 and 14223/1975 and Japanese Patent Disclosure No. 140237/1978 require a metal selected from the group consisting of ferric iron, magnesium, and manganese and mixtures thereof as well as the zinc which is a major ingredient of the coating composition. Japanese Patent Publication No. 4324/1965 requires glycerophosphoric acid as an essential ingredient.
Thus, all of the prior art coating compositions containing a complex fluoride ion are substantially different from the compositions of the present invention.